class

dir

id

lang

lang

lang (as an attribute name)
denotes an attribute whose value
is a language code for the natural language of the content of
any element; its value is inherited. This name is reserved
by virtue of its definition in the XML specification.
Notes
Attempting to install the relevant ISO 2- and 3-letter
codes as the enumerated possible values is probably never
going to be a realistic possibility.
See BCP 47 at
http://www.rfc-editor.org/rfc/bcp/bcp47.txt
and the IANA language subtag registry at
http://www.iana.org/assignments/language-subtag-registry
for further information.
The union allows for the 'un-declaration' of xml:lang with
the empty string.